The present invention relates to a method for controlling a continuously variable transmission (hereinafter referred to as CVT), and particularly to a method for controlling the speed ratios of the CVT.
A CVT is driven by a V-belt extending between driving and driven pulleys, with one or both of the pulleys being able to adjust the size of their respective V-shaped openings which hold the V-belt, thereby controlling the revolution ratio between the driving and driven pulleys. The size of the V-shaped opening is varied by changing the amount of pressure being fed into or drained from a hydraulic cylinder of either of the pulleys. The effective diameter of the pulley, therefore, can be varied by changing the amount of pressure being fed into or drained from the hydraulic cylinder, thereby allowing for a continuously varying ratio of driving pulley diameter to driven pulley diameter. The V-shaped openings on the driving and driven pulleys are each defined between a fixed member of the pulley and a movable member of the pulley. The size of the V-shaped opening is dependent upon the pressure inputted to or released from the hydraulic cylinders. A hydraulic apparatus supplies hydraulic pressure to each of the movable pulleys. advantage of such a CVT is that it can obtain a minimum fuel consumption rate.
To improve the control of the CVT, it has been proposed to set a control such as desired engine speed RPMo to be a function of a throttle opening .theta. and a vehicle speed V, (U.S. application Ser. No. 574,739, Filing Date Jan. 30, 1984, Title of the invention: METHOD FOR CONTROLLING A CONTINUOUSLY VARIABLE TRANSMISSION). According to the control of the prior application, the desired engine speed RPMo is determined regardless of the limit within which the speed ratio of the CVT can be controlled. Hence, when a throttle opening is small, an actual engine speed RPMe is lowered to the desired engine speed RPMo even after the movable member of the driven pulley contacts a stopper and the speed ratio e of the CVT reaches the maximum value within which the speed ratio e can be controlled. To control the movable member, hydraulic fluid is continuously applied into the hydraulic cylinder of the movable member of the driving pulley. Hence, the tensile force of the belt is forced to increase by the reaction of the stopper. This is reduces the life of the belt.